It is to most end users’ benefits to have the ability to work in an off-line mode. We frequently see people working away on reports or email while traveling on airplanes or working in places that do not have internet access.

End users have the option to work in an off-line mode while working on their email. They are able to do this because Outlook supports files known as dot OST (.OST) files. These dot OST files are Offline Folder files that allow users to make changes to their email files and then, at a later or more convenient time, synchronize those changes with the Exchange server.

But sometimes they may encounter performance problems when they are working with items in a large dot OST file. One of the problems they may have is that Outlook stops responding or hangs during normal usage. End users will more than likely be in the middle of an email activity such as reading emails, saving emails or deleting emails.

Earlier versions of Outlook, prior to Outlook 2007, did not exhibit these problems. Most often the slow response or no response problems are with very large dot OST files that are greater than four Gigabytes (4GB). Slow response times can be seen during email downloads or during synchronization and will often take longer and longer as the dot OST file continues to increase in size. Administrators should note that the slower the disk speed is then, the more likely it is that the slower response times will be seen.

Microsoft made changes when they introduced Outlook 2007 and some of those changes contributed to the slow response time. One of those changes occurred in how data was written to disk. As the number of items in the dot OST files increased then how often data was being written to the disk drives was also increased. Part of this methodology was meant to accommodate the new data structures of the dot OST files.

Microsoft released an update (933493) which can be found in the Microsoft Knowledge Base to correct this problem. Once the update is installed your systems should see a general overall improvement in response times during operations which previously lagged and sometimes caused Outlook to hang. Any email activities such as reading email, saving email, deleting and downloading email will be much more improved as far as response times are concerned. The update specifically improves and reduces the time involved when writing data to dot OST files on disk.

Several performance improvements were implemented with this update most of which include the following:

• Reduces the time required to copy or to move messages from one folder to another.
• Increases the download speed.
• Reduces the time required to delete messages.

Other options are available which can be used to maximize the response times for Outlook. These options include:

• Reduce the size of the Exchange mailbox.
• Use an online mode profile instead of a cached mode profile.
• Use a filter to restrict the items that you synchronize from Exchange to your dot OST file.

For this last option it is very easy to create a filter to minimize the number of items that you wish to be synchronized. Basically you can follow the steps outlined below:

1. Choose your dot OST folder
2. Right click the folder and then select Properties.
3. A Properties dialog box will pop up.
4. Click the Synchronization tab.
5. From the Synchronization tab, click Filter.
6. A Filter dialog box will pop up.
7. Indicate the criteria to be used for synchronizing new items that you want to arrive in this folder.
8. Click OK or hit enter two times.

You’re done. What will happen next is that the current size of the dot OST file will be reduced. The next time Outlook synchronizes this file any items that used to be synchronized prior to this change will be removed from the local copy of the dot OST file. And if they do not match the filter criteria then they will be removed.

Manual compaction of the file – by clicking the “Compact Now” button in the Advanced settings of the Exchange account – will allow you to see the file size reductions as a result of these changes. Or you can wait for compactions to occur in the background.

If you want to see the items that are not synchronized with the dot OST files then you can use a profile that is configured to use online mode from another client or you can use Outlook Web Access.